Image registration may also be applied to image sequences, i.e., to more than two images. Such image registration is frequently used to compensate for motion occurring within the image sequence. For example, in perfusion cardiac MRI sequences, the respiratory motion of a patient may hinder a clinician in studying the image sequence. Image registration may be used to compensate for the respiratory motion, i.e., to stabilize the image sequence.
For example, a publication titled “Registration of Ultrasound Image Sequences for Perfusion Analysis”, by Vratislav Harabis et al, EUSIPCO, August 2011, pp. 1015-1019, describes a method for registration of images in ultrasound sequences when a contrast agent is administrated in the blood stream. It is said that that selection of a reference image in registration of the contrast sequence is one of the main problems. It is said that a first possibility for the selection is based on a floating reference image, where a second image is registered to a first image, a third image is registered to the second image, etc. A second possibility is to set only one reference image, and to register the other images in the sequence with respect to this fixed image. The publication proposes a different method based on registration of subsequences. The original sequence is automatically divided to subsequences based on contrast values. In each subsequence, one image with the highest value of a CROI metric is set as reference for the registration of the subsequence. The CROI metric is based on pixel intensity differences in a region of interest (ROI). Finally, mean images from each registered subsequences are computed and used for registration of subsequences together.
A problem of the aforementioned method is that the registering of an image sequence can cause distortions to occur in the registered image sequence.